Many types of wireless communication devices receive and process signals transmitted by more than one user. For example, a base station in the WCDMA (Wideband CDMA) uplink receives signals transmitted by multiple users. One or more users may transmit more than one signal. Devices that receive and process signals transmitted by different users are commonly referred to as multi-user receivers because they demodulate and decode a codeword extracted from each of the different signals. Signal transmissions by multiple users often overlap in time and frequency, causing interference at the multi-user receiver.
An optimal multi-user receiver demodulates all signals jointly, at the expense of enormous complexity. Suboptimal multi-user structures such as parallel or serial multi-user receivers are more practical solutions for cancelling multi-user interference in that they can provide suitable interference cancellation for certain applications with much lower complexity. Suboptimal multi-user receivers enhance performance by involving the individual signal decoders in the multi-user receiver. For example, in the WCDMA uplink, multiple users transmit in a somewhat uncoordinated manner. That is, signals arrive at the base station with their slots misaligned, different users may have different spreading factors, etc. Each user individually coordinates with the base station, but without any direct relation to other users.
The base station can boost its performance by cancelling signals it has already processed from the common received signal. Doing so improves the performance of subsequent signals. In a purely illustrative example, two signals may arrive at the multi-user receiver and overlap by T′ ms. The span of each signal contains all the samples corresponding to a respective codeword. In the WCDMA uplink, this is referred to as a TTI (transmission time interval). A slot duration is 0.67 ms in WCDMA and a TTI may include 3 slots (2 ms) or 15 slots (10 ms). As such, the spans of the two signals are not necessarily of equal length. The base station can cancel the first signal from the combined received signal to benefit the second signal.
However, the receiver for the first signal must wait until all samples in the corresponding span are received, which ends T−T′ ms later than the span of the second signal. After the receiver for the first signal is done with decoding, it then reconstructs the first signal for the purposes of cancellation. Ignoring processing delays for simplicity, the receiver for the second signal must wait at least an additional delay T−T′ms to benefit from the cancellation of the first signal. The delay incurred is related to the length of the codeword associated with the first signal and slows the feedback from the decoder to the cancellation stage. This delay is not acceptable for certain application such as voice. Delay-sensitive applications may prevent otherwise beneficial cancellation from being used. Furthermore, it is difficult to run a stable system where cancellation is turned on and off depending on random relative offsets between signals.